1. Field of the Invention
The present invention relates to a control method and a control apparatus for controlling both an output of a non-linear system (in which dynamic characteristics that vary corresponding to a control input) and dynamic characteristics (such as a viscous characteristic, an elastic characteristic, and mass (inertial force)) of the system at the same time and obtains a control input that simultaneously satisfies both target paths for the system output and the dynamic characteristics.
2. Description of the Related Art
A manipulator that is driven by artificial muscles is an example of a non-linear system.
Examples of the artificial muscles are pneumatic rubber type artificial muscles, wires, and so forth. The pneumatic rubber type artificial muscle is constructed of a rubber tube that is contracted by air pressure and thereby generates a tension. In the wire type artificial muscle, viscous and elastic characteristics are provided by feedback control. Since both types have variable viscous and elastic characteristics and produce only a tension, they are similar to natural muscles.
In a manipulator driven by artificial muscles, a pair of artificial muscles are disposed around each joint so that one artificial muscle is in contention with the other paired artificial muscle.
The difference of torques produced in the contended muscles becomes a resultant torque of the joint. In addition, when antagonistic muscles are operated at the same time, the viscous and elastic characteristics of a joint can be adjusted.
Thus, by controlling the viscous and elastic characteristics of each joint, the overall dynamic characteristics of the manipulator can be controlled. When a robot manipulator is operated, dynamic and mechanical interactions take place between the manipulator and its environment (for example, a workpiece). Examples of these interactions are deburring operations, part mounting operations, surface abrading operations.
The manipulator that performs such operations should have a compliant property to an external force applied from the environment so as to prevent the manipulator itself and the environment (workpiece) from being broken.
When the manipulator has compliance to an external force, it can be displaced while it produces a reactionary force against the force applied to it from the environment in the same manner that when force is applied to a spring, the length thereof varies.
The compliance of the manipulator can be represented by a mechanical impedance. The mechanical impedance can be represented by the apparent inertia, viscosity, and elasticity of an end-point (contacting edge) of the manipulator.
When a manipulator has a compliant property to the environment and the compliance thereof can be freely assigned, complicated operations such as parts mounting operations and human interfacing operations can be executed with simpler commands than those of conventional rigid type manipulators.
As means for providing manipulators with compliance, RCC (Remote Center Compliance) device, impedance control (including compliance control), manipulators driven by artificial muscles, and so forth have been proposed.
The RCC device is a hardware apparatus that provides a manipulator with compliance by mechanical elasticity such as springs. Since this apparatus uses springs, it is difficult to freely assign physical constants and positions of the springs. Thus, a dedicated hardware system should be provided corresponding to the operational environment.
In the impedance control method, a mechanical impedance is provided to the end-point (effector) of the manipulator by a controller.
In the impedance control method, since the compliance is accomplished by software, and although the compliance can be freely assigned, the control is difficult. This method has not been practically employed.
On the other hand, in the manipulator that is driven by artificial muscles, the compliance (dynamic characteristics) of the manipulator can be controlled by simultaneously operating antagonistic artificial muscles with variable viscosity and elasticity.
The artificial muscle type manipulator has the advantages of being flexible and light, and it is expected to be used for applications such as human interfacing operations (free from causing damage to human beings) and high place operations that cannot be performed by the conventional manipulators. However, the artificial muscle type manipulator has the following problems.
Firstly, since the artificial muscles only produce tension, they should be disposed around each joint so that they are in contention with each other. Thus, the number of the required artificial muscles is twice as many as the number of joints. Consequently, the control input to the artificial muscles has excessive degrees of freedom and thereby the control input becomes redundant.
Secondly, since the artificial muscles are sometimes non-linear systems, it is difficult to cause a required torque to be produced at a particular joint.
Thirdly, although the dynamic characteristics of the manipulator can be varied, a control method such as a calculation torque method using inverse dynamics (inverse model of an equation of motion) cannot be easily employed.
Because of such problems, the control of the artificial muscle type manipulator has been made only for a position control. The variability of the dynamic characteristics has just been verified. Practical methods for controlling and employing the artificial muscle type manipulators have not yet been proposed.